Fiber optic sensors employ the fact that environmental effects, such as pressure, strain, vibration, and temperature, can alter the amplitude, phase, frequency, spectral content, or polarization of light propagated through an optical fiber. Advantages of fiber optic sensors include their light weight, small size, passive nature, energy efficiency, and ruggedness. In addition, fiber optic sensors have the potential for very high sensitivity, and wide bandwidth. Yet further, certain classes of sensors can be distributed along the length of an optical fiber so that an appropriate interrogation system can be employed to monitor selected environmental parameters at multiple locations at the same time.
One type of distributed fiber optic sensing technique is referred to as distributed vibration sensing (DVS), also known as distributed acoustic sensing (DAS). DVS/DAS techniques employ an optical fiber as a very sensitive, continuous sensor of dynamic strain. In use, the fiber is coupled to vibration which applies strain to the fiber and this converts the fiber into a distributed sensor of mechanical vibration.
One example of the use of DVS/DAS sensor is in vertical seismic profiling. In general, seismic profiling employs seismic sources to generate seismic signals that are propagated into a subterranean structure. The propagated seismic signals are reflected from subterranean elements in the subterranean structure, where the reflected signals are detected by seismic sensors, such as a distributed fiber optic sensor. The data collected by the seismic sensor is then processed to determine characteristics of the subterranean structure. Other applications for DVS/DAS techniques including monitoring noise in a well that is caused by, for example, the movement of fluids and other processes occurring in the well, or by a mechanical equipment permanently or temporarily installed in the well.